This invention relates to anionic clay compositions having the hydrotalcite crystal structure and, more particularly, to anionic hydrotalcite clays which are essentially carbonate-free and contain a one or more pH-dependent anion such as a boron-containing anion or Group Vb or VIb metalate located interstitially between the positively charged layers of metal hydroxides, and a direct and simplified method of preparation of such anionic clays from their components.
Clay minerals are composed of infinite layers of metal or non-metal oxides and hydroxides stacked one on top of the other. In the case of the widely-found cationic clays, interlayer cations (Na.sup.+, Ca.sup.2+ etc.) charge neutralize the negatively charged oxide/hydroxide sheets. The far less common anionic clays have positively charged metal oxide/hydroxide layers with anions located interstitially. Many of these are based on double hydroxides of such main group metals as Mg and Al and transition metals such as Ni, Co, Cr, Zn, Fe etc. These clays have a structure similar to brucite [Mg(OH).sub.2 ] in which the divalent ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In these anionic clays some of the divalent ion is isomorphously replaced by a trivalent ion, say Al.sup.3+. The Mg.sup.2+, Al.sup.3+, OH.sup.- layers are then positively charged necessitating charge balancing by insertion of anions between the layers. One such clay is hydrotalcite in which the carbonate ion is the interstitial anion. Rhombohedral hydrotalcite has the idealized unit cell formula [Mg.sub.6 Al.sub.2 (OH).sub.16 ]CO.sub.3.4H.sub.2 O. However, the ratio of Mg/Al in hydrotalcite can vary between 1.7 and 4 and various other divalent and trivalent ions may be substituted for the magnesium and aluminum. In addition, the anion, which is carbonate in hydrotalcite, can vary in both the naturally occurring and synthetic varieties being replaced by a variety of simple anions such as NO.sub.3.sup.-, Cl.sup.-, OH.sup.-, SO.sub.4.sup.2- etc. in naturally occurring varieties and by more complicated pillaring organic, inorganic, and organic-inorganic ion combinations in synthetic varieties. Hydrotalcites containing the large pillaring anions are generally made by substituting a hydrotalcite containing a simple anion by the larger pillaring anion. Substitution techniques which have been used are ion exchange and acid treatment in the presence of the desired replacing anion. Through changes in the size of the pillar used to separate the sheets in the clay structure, the pore size of the clay may be tailored to a particular use.
Processes for making hydrotalcite clays have been the subject of a number of publications, See, for example, U.S. Pat. Nos. 4,539,306 and 4,539,195 which are directed to pharmaceutical uses for hydrotalcite. Miyata et al. in U.S. Pat. Nos. 3,796,792, 3,879,523, and 3,879,525 describe a large number of hydrotalcites with divalent anionic layer substitution and include hydrotalcites containing B.sub.4 O.sub.7.sup.2- and the transition metal anions CrO.sub.4.sup.2-, Cr.sub.2 O.sub.7.sup.2-, MoO.sub.4.sup.2- and Mo.sub.2 O.sub.7.sup.2-. Both compositions and preparative methods are described, and the compositions are said to be useful for catalytic purposes, absorbents, desiccants, and the like. Miyata et al. in Clay and Clay Minerals, 25, pp, 14-18 (1977) describe the preparation of MgAl hydrotalcites containing SO.sub.4.sup.2- or CrO.sub.4.sup.2- as the interleaving anion by (1) mixing a solution containing MgCl.sub.2 and Al.sub.2 (SO.sub.4).sub.3 with a solution of sodium hydroxide or (2) mixing a solution of MgCl.sub.2 and AlCl.sub.3 with a solution of sodium hydroxide and a solution of sodium chromate.
In U.S. Pat. Nos. 4,458,026 and 4,476,324 and Journal of Catalysis 94, 547-57 (1985), Reichle describes catalytic reactions including aldol condensations using synthetic hydrotalcites containing smaller anions and also large organic anions such as long chain, aliphatic, alpha-omega dicarboxylates. The materials are made by dropping the source of divalent and trivalent ions into a basic solution of the desired anion.
Miyata and Kumura in Chemistry Letters pp. 843-8 (1973) describe hydrotalcite clay materials containing Zn.sup.2+ and Al.sup.3+ with dicarboxylate anions and show that the interlayer spacing obtained using X-ray diffraction expands from 9.4 Angstroms to about 19 Angstroms as the dicarboxylate anion is changed along the series oxalate, malonate, succinate, adipate and sebacate. This study indicates the carboxylate anions are in the lattice standing roughly perpendicular to the layers.
Pinnavaia et al. in Synthetic Metals 34, 609-15 (1989) reports an EXAFS study of some Zn.sub.2 Al, Zn.sub.2 Cr and Ni.sub.3 Al hydrotalcites with pillaring anions such as V.sub.10 O.sub.28.sup.6- made by exchanging the hydrotalcite chloride or nitrate with a salt of the transition element metalate. Similar materials using the Mo.sub.7 O.sub.24.sup.6- and a series of Keggin-type ions such as .alpha.-[H.sub.2 W.sub.12 O.sub.40 ].sup.6- were also made by exchange.
Recently, two U.S. Pat. Nos. 4,774,212 and 4,843,168 have appeared, describing hydrotalcites pillared with large organic, inorganic, and mixed organic-inorganic anions made by first preparing an organic anion-pillared hydrotalcite at one pH, and then substituting partially or fully the organic anion by a large transition element metalate at a different pH. The patents describe the use of such materials for the catalytic dehydrogenation of t-butylethylbenzene to t-butylstyrene as well as other catalyzed reactions.
The success of molecular sieves for catalytic purposes has prompted a search for other porous inorganic materials which could act as shape selective catalysts. Pillared cationic clays have been investigated as part of this search but the small number of useful large cations and the amount of open volume left after completely pillaring many of the materials has been a concern. Also, the poor thermal stability of several of the cationic pillars has been discouraging since the pillars collapse during higher temperature catalytic use. Cationic pillared clays are usually acidic.
Anionic clays, which are usually basic clays, have also been considered, as has been reviewed above, but the work reported has resulted in few examples of anionic clays with substantial catalytic potential, i.e., a clay with a large interlayer spacing having an incompletely stuffed gallery containing useful catalytic sites open to reaction. Also, because of pH limitations on the hydrotalcite preparation, not many examples of such materials have been made even with small non-pillaring anions, and those hydrotalcites made so far with large pillaring anions are few in number and made by a time consuming process.
Now a simplified and direct technique has been found to produce hydrotalcite-type clays with sizable interlayer spacings that are pillared by pH dependent, essentially carbonate-free, boron-containing anions and transition element metalates. The technique extends the number of large anions which can be incorporated in a hydrotalcite-type clay and also the range of metal ions which can be substituted for the divalent and trivalent metal ions of hydrotalcite. Such hydrotalcite-type clays can have more open galleries, contain catalytically active ions in the anionic layers, and catalyze a wide range of hydrocarbon conversion reactions.